Method of making polymeric film

ABSTRACT

An apparatus is provided for making polymeric film wherein flowable polymeric material is extruded in molten form through an extrusion die orifice in the form of a web of film and which apparatus includes means for selectively controlling (by heating or cooling) the temperature of isolated parts of the die lips defining the die orifice whereby to improve thickness uniformity of the film. A method also is provided of making this polymeric film of improved thickness uniformity.

June 25, 1974 R. D, MULES 3,819,775

METHOD OF MAKING- POLYMERIC FILM Filed Aug. 15, 19''? 3 Sheets-Sheet 1JUNCT- ION I4 I 4 FIG. I /|6 T01 22W 3; a SPUOPWPELRY W A w ouincmucl[:j DIGITAL M V CONVERTER FIG.4 23

a WW l \l W U u June 25, 1974 R. D. MULES 1 METHOD OF MAKING POLYMERICFILM Filed Aug. 15 1972 3 Sheets-Sheet 2 Q 35 m M' I 2 June 25, 1974 R.D. MULES I 3,819,775

ammo OF MAKING POLYMERIC FILM Filed Aug. 15. 1972 3 SheQts-Sheet 5United States Patent 01 lice 31,819,775 Patented June 25, 1974 r3,819,775 METHOD OF MAKING POLYMERIC FILM Richard D, Mules, 4 WheelerCourt, Darley Woods, Wilmington, Del. '19810 Filed Aug. 15, 1972,'Ser.No. 280,890 'Int. Cl. B29f 3/08 U.S. 'Cl.'264-=40 3 Claims ABSTRACT FTHE DISCLOSURE "BACKGROUND OF THE INVENTION (1 Field of the inventionThis invention is .a method of an apparatus for making,,by extrusion, a.webv of polymeric film in which molten polymeric material is selectivelyheated or cooled adjacentthe-lips of an extrusion die in a novel mannerto obtain desired film, thickness characteristics.

(2) Description-of the Prior Art There have been numerous attempts toimprove thickness properties in extruded films. Numerous apparatus andmethods, for example, have evolved to sense thickness deviations andeffect adjustment of film thickness at specific locations usually at ornear the point of extrusion of the polymeric material in the filmforming operation.

For instance, US. Pat. 3,122,782 describes a mechanical arrangement foradjusting the thickness of selected hypothetical lanes. across a filmstructure corresponding to thickeror thinner portions of the film. Insuch patent, two interconnected multiple-junction terminal boards aredisposed intermediate a film thickness sensing gauge and an adjustmentmeans for changing the width of the extrusion orifice from whence thefilm structure issues. The film thickness gauge measures the thicknessof the film structure and transmits a bias signal through suitable relayelements for energizing motors which are so constructed andarranged tocause opening or closing of the orifice opening in the extruder inaccordance with the deviation in .the thickness of the film that issensed by the film thickness measuring gauge.

V A method of improving the thickness uniformity of polymeric film isdescribed in US. Pat. 3,161,711 which disclosesheating selectedlongitudinal lanes of greater thickness of amorphouspolymeric filmsfollowed by cooling the entire film whereby to effect a more markedtemperature differential between the thinner and the thickerlongitudinal lanes of the filmstructure, followed, lastly, by stretchingthe unsupported film structure thereby to obtain afilm structure ofimproved thickness uniformity. In US. Pat. 3,455,175, an extrusionapparatus is disclosed with adjustable lip members containing'aplurality of adjustment screws along the orifice slot. The screws arearranged closely together on about 20 mm. (.787 inch) spacing andarethreaded into the lip members which have been modified near. theextremities by locally weakened cross sections. Manual rotation of thesmall diameter -screws',resultsindisplacementof the lip members and. bythat means-the slot opening defining the thickness of the web is varied.Confining the deflection of a lip member to a narrow area approximatingthe width of a single adjustment screw is most difficult due to theinherent interaction of adjacent screws. Each time the setting of ascrew is changed, there is a corresponding unavoidable disturbance ofthe setting of neighboring screws resulting in an unpredictabledeflection of the lip members. To counteract the undesirable deflection,thesettings of the neighboring screws must also be corrected. As aconsequence, narrow band or streak nonuniformities cannot easily beeliminated without upsetting the basic gauge setting of the lips. As isapparent, adjustment means of this sort require constant personal attention and trial and error manipulation by skilled operating personnel.

Conversely, the difficulties of reliance on skilled operators and trialand error control are to some extent obviated by an automatic gaugecontrol system, such as is disclosed in US. Pat. 3,122,784, whichfeatures a plurality of speed controlled adjuster motors mechanicallycoupled to displaceable lip segments on a web extrusion apparatus. Themotors respond to signals generated by a traversing thickness sensingmeans located at some distance below the extrusion apparatus whichmeasures the final thickness of the web from standard referencedeviations. Electrical signals are thus generated, translated intothickness corrections and distributed by way of switching meanssequentially to the input of the motors. This system cannot, however,eliminate narrow streak and gauge band nonuniformities because theindividual adjustment means control relatively wide segments of theorifice slot. Moreover, the nature of the nonuniformities is often toosubtle and complex for corrective adjustment by mechanical displacementof die lip members.

In US. Pat. 3,161,711 is disclosed a process for improving filmthickness uniformity properties which includes heating a substantiallyamorphous thermoplastic polymeric film while substantially free fromtension along at least one selected longitudinal lane of greaterthickness than the adjacent thinner portions of the film and cooling thefilm to sharpen the temperature differential between the thinner andthicker portions, followed by stretching. The heater is positioned afterthe quench drum and preheat rollers and before the film passes into thefirst set of nip rolls.

In U.S. Pat. 3,649,726 there is described a thickness control apparatusand method for polymeric film structures including means for and stepsof measuring the thickness of the film after stretching, computing thecumulative mass values of the film before and after stretching andadjusting the thickness of the film structure before stretchingresponsive to deviation from a preselected value in thickness afterstretching corresponding to the cumulative mass value of the film beforeand after stretching. The thickness of the film structure is adjustedbefore stretching by varying the opening of the orifice lips or byheating means disposed intermediate the cooling means and the stretchingmeans.

And, lastly, US. Pat. 3,341,388 discloses a method of an apparatus forextruding thermoplastic film wherein heaters are positioned in the diebody whereby to heat those" areas of the die'to help maintain thethickness of the sheet substantially constant. i

While these patents and others show polymeric film thickness-controllingor related devices, they do not show controlling the temperature ofisolated lip parts to obtain better thickness properties in the film, asin the instant -invention.

SUMMARY OF THE INVENTION This invention .is a method of an apparatus forcontrolling polymer'fiow prior to extrusion in making a web of polymericfilm having improved physical properties.

In the manufacture of film for a variety of uses, it is known to improveits characteristics by controlling flow of the polymeric material in theextrusion apparatus. Nonetheless, in making such films by many knownmethods, the film, as formed, usually has one or more zones or areas ofgreater or lesser thickness than the rest of the film (i.e., gaugevariations) which are caused by polymer flow imperfections orvariations, which, in turn, create nonuniform patterns of the polymericmaterial in the extruding device.

In the extrusion of polymeric materials in web form, it is known thatgauge or thickness of the film, its unit weight and the roll formationcharacteristics of the web, sheet or tube, are all directly controlledby or determined by the uniformity of flow of the polymeric materialthrough the extrusion die and other parts of the extrusion apparatus,particularly the die lips. For example, when the web is wound, anycontinuing, standing gauge variation would lie one atop the other sothat in cumulative effect a hard hand is built up, which distorts theroll, thereby impairing its utility and causing waste.

The manufacture of thin gauge polymeric film, for example, from orientedpolyester, polyethylene and polypropylene material, usually commenceswith the formation of a relatively thick web of material extruded in athermoplastic condition from a narrow elongated extrusion orifice of afiat plate extrusion die. The molten polymeric material is depositedonto a smooth, cooled, endless moving quenching surface, such as astainless steel conveyor band or a rotary drum and solidified. Thehardened material in the form of a web of film is subsequently removedfrom the cooled surface and forwarded into a stretcher apparatus whereit is reheated to just above its glass transition temperature andstretched longitudinally and transversely to the desired thickness. Fromthere, the thin gauge web of film is sent to other processing stationswhere it may be given a variety of surface treatments depending on theend use, then suitably slit and wound into packages or rolls, inaccordance with customer specifications.

The superior physical properties of oriented polyester material are nowbeing exploited in the form of high strength ultra-thin films. Thesefilms, ranging from 0.0002 to 0.0010 inch thickness, are findingwidespread use in magnetic recording tapes, electrical applications,surface coverings and general thin film packaging. As the material ismade in the ultra-thin range, it has been found that gaugenonuniformities become increasingly critical in the formation of a highquality product. Specifically, small variations such as narrow gaugebands and streaks heretofore tolerable are now exceedingly troublesomeand defy correction on conventional extrusion apparatus by mereadjustment of the die lips, for example. The origin of these streaks andgauge bands can be traced to numerous sources that in some instances mayinclude the polymer preparation and transfer systems as well as theextrusion die apparatus itself. Still other sources of thesenonuniformities are thought to be the result of sporadic surges in thepumping system, uneven heating of the molten material and, even,inherent differences in the composition of the polymeric material.

This invention is directed to a method of an apparatus for making filmby extrusion and, more particularly, to a method of an apparatus forcontrolling the temperature of molten polymeric material in a novelmanner in the extruding device by the use of novel isolated die lipparts which assures controlled flow of the polymer and also substantialfilm thickness uniformity.

By following the method of this invention and by using the novel polymerflow controlling device thereof, polymeric films of excellent propertiesand gauge uniformities may be produced capable of meeting untold numbersof use requirements.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows an extrusion die of thisinvention with vernier heated die lips tied into an automatic closedloop gauge control system (with parts omitted for clarity) forcontinuous high productivity operation. The system includes a traversingweb thickness scanner, an analog-todigital signal converter, atemperature feedback means on the heated die lips, a power supplyinterface and a digital computer. In the system, the computer receivesweb thickness variations and temperature readings from the individualheated parts of the die, differentiates the variations relative to adesired thickness profile and initiates corrective signals to the powerinterface which regulates electrical power to the heater elements.

FIG. 2 is an isometric view of a preferred embodiment of the extrusiondie of this invention (with parts omitted for clarity) showing thermaldie lip parts separated by dead air spaces and minimum thicknesssections along the die lips which serve thermally to isolate the thermallip parts from each other and the main body of the die.

FIG. 3 shows a typical thermal boundary layer formation through themolten stream of polymeric material as it moves, unidirectionally,through the controlling temperature zone wherein the heaters arepositioned adjacent the die lip surfaces.

FIG. 4 is a partial front view showing a close-up along the ends of thetop and bottom body members.

FIG. 5 shows an extrusion die of this invention with full web widthpinning means, along with known parts of a film making apparatusincluding machine and crossmachine film stretching means and web windupmeans.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, thereis shown an apparatus of this invention for feeding fiowable moltenpolymeric material and extruding it in the form of a web of polymericmaterial to provide film, such as polyethylene terephthalate film,having enhanced psyhical properties.

Referring to FIGS. 1 and 3 in particular, it will be seen that polymericmaterial M is supplied to the apparatus of this invention from a supplysource, not shown, and moved or fed into a die adapter through which thematerial flows to a novel extrusion die 10 of this invention, from whichit is extruded in the form of a web W of polymeric material.

The extrusion die 10 includes die lips 11 which define an extrusionorifice 12, with each lip 11 including a plurality of closely spacedheated lip sections or parts 13. Each lip part 13 includes a heaterelement 14 and a temperature sensor 15 'which is connected to anindependently controlled by a regulated power supply 16 so that each lippart 13 is substantially thermally isolated from the other. Prior to aproduction run, the area of influence of each thermal lip part 13 on theweb W of polymeric material is identified and calibrated. Power level toselected heated lip parts 13 is then manually adjusted by an operatorwith the aid of a suitable web gauge measuring device 17. One suchdevice commonly used and commercially available is a beta-ray scannerwhich is mounted on a traversing mechanism located at some distancedownstream of the extrusion die, preferably at a point after the web Wof polymeric material has been solidified into a web of film F. Thescanner is moved back and forth a cross the width of the web of film Fand records thickness variations which can be displayed conveniently asa thickness profile on a cathode ray tube. By reference to the profiletrace, the operator makes appropriate adjustments to the power level ofthe heaters influencing the area of the gauge variation.

The adjustment of the power level can be either to increase or decreasethe temperature of the selected thermal lip part 13 in accordance withthe type of defect being dorrected. By adjustment of the power levelupward, the temperature of the molten stream opposite the selectedthermal lip part 13 is raised. This causes a localized delcrease'in themelt viscosity of the molten material M and a corresponding localincrease in the mass flow rate. Be-

cause the residence time of the material in contact with thehightemperature zone'is very brief, no significant degradation of thepolymer occurs.

Depending on. the composition and behavior of the polymeric materialandthe desired gauge correction, the

heating geometry can be tailored either to increase or to -ing alocalized increase in the relative viscosity of the material and acorresponding localized decrease in the mass flow rate. Gauge correctioncan be obtained by either heating or cooling the thermal lip parts 13with good results.

The gauge control concept is particularly Well suited for use in anautomatic closed loop system controlled by a digital computer previouslyprogrammed with a desired web thickness profile. As shown in FIG. 1,such a system includes the aforementioned extrusion die 10, a PDP-8 /Ecomputer 18 made by the Digital Equipment Corporation which is connectedthrouhg analog-to-digital converters 19a and 19b to the output oftemperature sensors 15 and the beta-ray scanner 17. Computer 18 receivesthe input data, compares the information with the desired thicknessprofile previously programmed into its memory and calculates the amountof electrical power that needs to be supplied to the vernier heaterelements 14 to achieve the desired gauge profile. The power isdistributed by the. computer 18 through a power supply interface 16which is connected to the heater elements 14 and to a standard 60 cycle,three phase, 220 volt alternating current source.

.Power supply interface 16 is connected to the heaters 14 and to thecomputer 18 through solid state switching circuitry which enables thecomputer 18 to regulate the power level individually and collectively tothe heaters 14.

In a preferred method, the computer 18 is programmed to distributethrough the power supply interface 16 an initial uniform power level toall the vernier heaters 14. This establishes a heated zone 21 havingsuface temperatures adjacent the molten polymeric material M about 10 C.above the entrance temperature of the material. As best shown in FIG. 3,movement through the heated zone 21 produces a boundary layer 22 alongthe top and bottom surfaces of the viscous material M and a higher localmass flow rate. Where a gauge band or streak develops, the controlsystem corrects the defect as follows: The computer 18 receives both thethickness variation signal from the scanner 17 and the temperature fromthe thermal lip part 13 opposite the defect. The computer 18 thencompares the variations in thickness with the desired programmed profileand calculates the amount of correction in the power level necessary tochange the mass flow rate of the material M in the defective region inaccordance with previously determined values. A power level signal isthen transmitted by'the computer 18 through the .power supply interface16 to the selected heater element 14. As previously mentioned in thepreferred method, the corrected power usually amounts to a reduced inputwhich has the effect ofcoolingthe thermal section 13. This cooledsection 13 produces a localized cooling of boundary layer. 22, acorrespondingincrease-in the viscosity of the material and a reducedlocal mass flow rate.

stantially across the extrusion orifice 12. The method of smoothing outgauge bands will depend on the amount of gauge correction, the existingset of extrusion conditions, including extrusion rate, draw rate andaccuracy of gauge. Thus, it will be seen that surface defects(resembling peaks and valleys) are readily corrected by either heatingor cooling the thermal sections 14, selectively.

In the preferred embodiment the extrusion die 10 comprises top andbottom body members 23 and 24 which are essentially rectangular-shapedblocks made of a dimensionally stable metal alloy suitably doweled andbolted together along a common interface, not shown.

Along the interface, the die 10 is provided with a polymer passageway 29that terminates downstream as the narrow elongated extrusion orifice 12.The passageway 29 consists, in sequence, of an inlet and distributionheader, not shown, a flattened preland 31 and, finally, a constrictedland 32. The distribution header, of a type known to the art, performsthe function of distributing the molten polymer across the width of thedie. Preland 31 performs the function of insuring substantiallyunidirectional fiow (i.e., machine direction flow) of the material Mprior to extrusion; while the land 32 establishes the basic gauge of theweb W of material extruded through the extrusion orifice 12. Byestablishing such unidirectional flow in the preland 31, the subsequentselected heating of the required lane of the molten material is madepossible since its location can be determined by in-line reference tothe deviating lane in the film.

The passageway 29 is connected to a continuous source of molten polymer(not shown) from which suitable polymeric compositions such aspolypropylene, polyethylene, polyamides and, preferably, polyesters arereadily processed through the die.

The extrusion orifice 12 essentially is defined by inner surfaces 35'and 36' of the die lips 11 (for clarity the upper and lower die dips arealso designated as 35 and 36) which, in turn, define the preland 31 andthe land 32. Adjustment bolts, as known to the art, may be used forpreliminary gauge adjustments, if required.

Each die lip 35 and 36, as previoously described, is

divided into a plurality of closely spaced lip thermal sections or parts13 each containing A" diameter cartridge-type resistance heaters 14.Each thermal lip part 13 is separated or isolated from its neighboringlip part by a narrow open slot or air space 41. Adjacent each heater 14is the thermocoupletype temperature sensor 15. In the preferredembodiment, a 20-inch long orifice 12 contains fifty-two thermalsections 13.

As best shown in FIG. 2, each lip thermal part 13, for all intents andpurposes, is thermally isolated from other lip parts by lip partsisolating means (i.e., the slots 41) which extend into the lips 35 and36 until only a thin rib or lip section connecting means or parts 42connects the adjacent lip parts 13 thereby defining the continuous lipsurfaces 35' and 36'.

At the base of each isolated lip part 13, the air spaces or slots 41terminate into enlarged circular apertures 43, each of which isseparated from the other by a thin rib 44.

The connecting parts 42 maintain the lip parts 13 as an integral part ofthe respective lips and assure that a smooth continuous surface 35' or36' is exposed to the molten material M. Conversely, the ribs 44 serveas heat dams to reduce the heat flux into the main body of the die. Thisis a critical feature of the apparatus and method of this invention forwithout the interposed dead air spaces 41 and the reduced ribs orconnecting parts 42 and 44 the outputs of the heater elements 14 wouldbe rapidly dissipated into the main portions of the body members 23 and24 of the die and into adjacent lip parts producing diminished laneresolutionand prohibitively long response time. With the aforementionedconfiguration, the temperature of a typical thermal lip part 13 can bevaried by as much as 25 C. before a 1 C. rise is attained in theadjacent lip parts.

The specific conditions employed in the extrusion apparatus of thisinvention depend on the nature of the material being extruded, as areknown to the art. For example, in extruding polyethylene terephthalatematerial, the conditions required for doing this are set forth in US.Pat. 2,828,421, which patent is incorporated herein by reference.

While the extrusion apparatus described above can be satisfactorilyemployed in casting a web of film at slower rates, higher productionrates may require selected pinning means as adjuncts for effectivequenching and gauge profile control, as described in copendingapplication (F-2396) Ser. No. 280,888, filed Aug. 15, 1972, nowabandoned. In the invention of such application, the web of material Wis fully pinned across its width, as by an electrostatic pinning wireextending across the full width of the web, as disclosed in US. Pats.3,223,757 and 3,068,528, or by a pneumatic pressure pinning method asdisclosed in copending patent application Ser. No. 232,763 of Huskey etal., filed Mar. 8, 1972. In methods employing these types of quenchingassists, generally designated 50, the web W is sufficiently wellanchored throughout its width to a typical moving quenching surface S(i.e., to quenching surface S of quenching drum 40, as shown in FIG. toprevent any transverse movement of the polymer mass. This assures thatthe location of the proper lane or lanes in the molten material may beaccurately determined by in-line reference to the deviating film lane orlanes. Thus, the response to the corrective action of a thermal gradientis essentially that which would be expected from viscosity changes inthe flowing polymer.

The following examples show quantitatively the degree of gauge controlobtainable by the vernier heated lip concept. Each example describes aseries of tests made with a standard fiat sheet extrusion die similar toa Series 102 unit manufactured by F. W. Egan Company. The die wasconnected to a continuous source of molten polyethylene terephthalatepolymer and operated in a normal manner in conjunction with a chilledrotary casting drum. The vernier heating concept of this invention wasonly incorporated at the mid-center region of the upper die lip asthirteen thermal lip parts spaced inch apart. Each lip part or sectionwas thermally isolated from the next in the manner previously describedand was adapted with a cartridge type heater and thermocouple sensor.The heaters were connected to independently controlled power unitssimilar to the type manufactured by the Electronics Control'Systems,Inc. At start-up, prior to measurement of the film, the extrusionorifice opening of the die was mechanically adjusted to a 60-m'il gapheight. Polymer throughput was adjusted at 400 pounds per hour under ashear rate of 230 reciprocal seconds across the land portion of the die.The drum on which the molten web was solidified was adjusted to a speedof 31 yards per minute so as to induce an approximate :1 draw ratio andthereby form a 0.007 inch thick cast film. These settings were then heldconstant for each test run which are described below.

EXAMPLE 1 Initial casting of the film was commenced with the heaterpower ofi so that the die and die lips were stabilized at 285 C. withpower to the vernier heaters off, A profile trace of the cast filmthickness was determined. The trace was obtained by an electromicrographtype thickness gauge similar to the type manufactured bythe Pratt &Whitney Company. Heaters H-l through H3 were then energized raising thetemperature of each heated lip part to about 20 C. above the initialtemperature of the molten polymer. After the system stabilized attheseconditions, a gauge profile trace was made of the'cast film. :Thepower level to heaters H-5 through H-9 ,was then, adjusted to increasethe temperature of the correspondinglip part 1 0 C. Following astabilization, period of 10 to 15 minutes, a profile trace of the castfilmwas made. The power level to heaters H-5 and H-9 was againreadjusted raising the lip temperature another 10 C. increment andanother profile trace of the cast filrnwas made. The above procedure wasrepeated at 10 C. intervals until a final lip temperature of 400 C. wasattained., A fter. obtaining a final profile trace, the power level toheatersI-I I through H-13 was then cut back, loweringthe temperature ofthe lipsto315C.

Analysis of the profile traces showed that the.0.'007 inch thick castfilm increased at the mid-section (directly opposite the heated lipportion) in thickness 0.00093 inch, a 13.3% increase over a 115 C.change in temperature. This represents an average change in gauge of0.11% per C. temperature rise that can be induced by heated lips. Thetest runs also revealed the rate of heating was nonlinear and rapid witha rate up to 60% goal temperature taking place at 0.344 C. per secondand from 60% to goal temperature at a rate of 0.177? C. per second.Conversely, the tests showed the rate of cooling was considerably higherthan the heating rate; When;the power to the heaters was reduced, thetemperature of the lips cooled at a rate of 0407' C. per second at 60%goal temperature and 0.252 C. per second at 90% goal temperature. Thefaster cooling rateis believed to be attributed to the high heatabsorbing capacity of the molten polymer stream.

EXAMPLE 2 This example illustrates the narrow of fine lane resolutioncapability of the heated lip concept of this invention. The equipmentandcasting conditions were substantially identical with those of Example1.

An initial gauge profiletrace of the cast film was made with the heatersoff after the die and die lip temperature was stabilized at 285 C. forcontrol purposes. Power to heaters H-l through H-13' was then turned onand the temperature of the lip elevated to about 330 C. A profile traceof the cast film was then made after the system stabilized. Power toheater H-7 was then raised in incremental amounts so that thetemperature of the individual thermal section continually increased.Simultaneously, the temperature of thermal sections-corresponding toheaters H6 and H8 was monitored. The temperature of .the heater H-7thermal section was raised until a 1 C. change in temperature of heatersH-6 and H-8 was detected. This test shows that a temperaturedifferential of 25 C. could be imposed between adjacent thermal lipsectionsor parts without a disturbing influence greater than 1 C. Thus,at a typical heating rate of 0.11% to 0.22% per C, it is possible toinduce a 2% to 5% variation in gauge co'rrection from lane to lane.

I claim: I I 1. A method of improving the thickness uniformity of a webpolymeric film including the steps of H transforming moltenpolmericmaterial within an extrusion die into a unidirectional flowstream;

passing the strearri betweenla n arrow temp'er'ature zone wherein thezone is adjacent the extrusion orifice of an extrusion die having dielips including a plurality of closely spaced,'- ther'nrally isolated andindependently controlled lip parts; i I I extruding the molten polymericfrom the extrusion orifice in the form of a web of polymeric film; and

selectively varying the temperature of at least a lip part therebyselectively to control the temperature of the molten polymeric materialin response to measurements of web thickness nonuniformities so as toalter the mass flow rate of the molten polymeric material in therequired areas and thereby improve gauge unilormity of the web ofpolymeric film.

2. The method of claim 1 wherein such lip part of the die lips is cooledwhereb to cool the molten polymeric material along at least a selectedlane thereof.

3. The method of claim 1 wherein such lip part of the die lips is heatedwhereby to heat the molten polymeric material along at least a selectedlane thereof.

References Cited UNITED STATES PATENTS 9/1973 Richardson 264-210 R9/1967 Bunyea 156244 US. Cl. X.R.

